Root-canal work often requires that a tool, such as a file, be inserted no more than a specific maximum distance inside the root of the tooth. Since it is physically impossible for the dentist to see what he is doing he must determine by other means just how far the tool has entered the root. To this end a full-scale X-ray is taken and the insertion distance is measured on the developed X-ray.
It is possible either to use a tool having a shaft extending beyond the handle by the predetermined maximum length, or more commonly, to fit an adjustable stop on the shaft. Such a stop may be a simple block of elastic material, or may be constructed of a housing and a compression spring as described in my above-cited patent application. In the latter case it is frequently necessary for the dentist to fit the stop to the tool while the patient is sitting there with his mouth held open. Thus it is mandatory that the operation be carried out with maximum speed. At the same time it is essential that stops be placed with perfect accuracy, as otherwise the possibility of poking beyond the tooth into the jawbone is presented. This fitting operation is particularly difficult when a relatively hard-to-slide stop as described in my above-cited copending application is employed. Traditionally the dentist simply pulls the stop along the shaft to what appears to be the proper location, then measures the length of shaft protruding and adjusts the position of the stop in several operations until it is correct. Obviously such a method is time-consuming, especially when following the lengthy operation of finding a stop properly sized for the tool in question.